The present invention relates to controlling the level of a pourable particulate material in a defined space, and more particularly to a method of effecting such control and an apparatus for carrying out the method.
There are many instances, for instance grinding mills which grind grain or the like, to name only one example, where it is necessary to control the level of a pourable material (e.g. grain).
One prior-art proposal suggests the use of bodies which vibrate at high frequency and which, when they are surrounded to a greater or lesser extent by pourable material, experience a damping of their vibration. This causes a signal to be triggered which indicates that the level of pourable material in the space in which the vibrating body or bodies are located, has reached at least the level of the body or bodies. However, this signal which can be so derived is only a digital signal and cannot be used for an analog regulation of the control of the pourable material. To obtain an anolog regulation a large number of such bodies would have to be provided, which would make for a complicated construction aside from the fact that it would not permit continuous measurement but only stepwise measurement.
A further prior-art proposal suggests to use an arm which swings about a horizontal axis and which contacts the pourable material so that, when the level of the pourable material rises, the arm is lifted, analogously to a float-control for sensing the level of liquid. When the lifting movement of the arm is fully completed, an end switch is actuated which produces a signal. However, this signal is again only of a digital nature, which is undesirable. Moreover, this prior-art device will operate only when the pourable particulate material, such as granules, powder or the like, is properly supported from below; if the material is continuously replenished from above and continuously withdrawn from below, there is a movement in downward direction in which case the arm will not be properly supported and the indication will not be accurate. A further prior-art proposal such as the use of a feeler which is mounted on a horizontal shaft and connected with a yieldable drive. The drive is intermittently activated to turn the feeler about the horizontal shaft and, if in so doing the feeler encounters pourable particulate material, the movement of the feeler is blocked and the drive yields. If no material is present, then the feeler will activate a switch which causes the addition of a predetermined quantity of further pourable material into the space. However, the difficulty with this prior-art arrangement is that it can be used only in certain circumstances, and does not permit continuous control of the level.
A further development of this last-mentioned prior-art proposal suggests that when the drive is activated, the displacement of the feeler can be measured so that the extent of such displacement indicates the level at which particulate material is present in the space in question. This, however, is also not suitable for a continuous control and, even if attempts were to be made to further develop this construction to provide a continuous control, the results would necessarily be unsatisfactory because the measurement would not be continuous and would take place only at the end of a stroke of the feeler. This means that the regulation would be substantially influenced by accidental fluctuations in the level of the particulate material because the next measurement would take place only at the end of the next stroke of the feeler.
Other prior-art proposals have also been made, but none of them have been entirely satisfactory. In the case of roller frames used in grinding mills and the like it has been proposed to use very sensitive elements to which the weight of the particulate material is transmitted. However, as soon as any significant amounts of the material should happen to adhere to these sensitive elements, the measuring results will be inaccurate. It has therefore been proposed to provide feelers which are pivoted about a horizontal axis and are loaded by a lateral pressure acting upon them in the region of the base of an essentially conical heap of pured particulate material. The thus-exerted force was found, however, to be insufficient to produce a reliable signal. Optical elements for controlling and supervising the level of the material, on the other hand, were found to be inaccurate because such material tends to develop dust which disadvantageously influences the accuracy of measurement, and devices based upon the use of radiation are complicated and in some instances are not entirely safe.